As emerging observations point to an increased rate of expansion of the universe[i], it appears our universe that will continue to expand indefinitely - essentially a universe without end. However, whilst the dimensions of the universe may know no boundaries, its contents are subject to a myriad of physical processes that will give rise to significant changes to the universe as it ages, and may eventually lead to a state where no further physical processes are possible, a point that may allow the universe to be declared at an end.
Following the Big Bang, slight inconsistencies in the distribution of matter led to the formation of galaxy sized structures that were the precursors to today’s galaxies. The matter that made up these infant galaxies could take several courses. It may float freely, not aggregating into distinct objects (hydrogen clouds). Alternately, it may suffer local collapse and form an independent body. If that body has sufficient mass, it may trigger hydrogen fusion in its core and become a star. Or it may simply become or storage vessel for hydrogen and helium (termed a brown dwarf[ii]). Whilst capturing a significant portion of the galaxy's mass, the majority of a star's mass will be recycled through super novae explosions or mass loss through solar winds, depending on the mass of the star[iii], giving rise to new stars, as well as other objects, such as planets, and by extension life. Whilst most stellar matter is recycled at the end of a star’s life cycle (the larger the star, the greater the proportion of mass lost[iv]), some remain in stellar remnants, which depending on the mass of the initial star, may take the form of white dwarfs, neutron stars or black holes. Thus, during a galaxy's life span, it will consist of the following object:
- brown dwarfs
- stars
- white dwarfs
- neutron stars
- black holes
- planets, comets, asteroids…
- free floating hydrogen and dust clouds[v]
This scenario approximates to the current state of our galaxy, and the fate of these objects will contribute to the fate of the galaxy. Most dynamic are the stars, which will continue to be created, and go through their life-cycle while ever fuel (primarily hydrogen) is available in sufficient quantities and densities. Smaller stars burn fuel more slowly, and the smallest stars will take 1013 years[vi] to burn their fuel and become white dwarfs[vii]. Eventually, however, all star will run out of fuel, and galaxies will not have enough fuel to create new stars. This loss of fuel represents the internal aging of galaxies. However, galaxies do not exist in isolation, but rather form clusters (galactic clusters), that are gravitationally bound. The motions and interactions of galaxies in galactic clusters create collisions between galaxies, and gradually cause the component galaxies to combine to form a single ‘meta-galaxy’[viii]. This may cause a rejuvenation of star formation, as the shock waves from the collision trigger star formation (although some argue that collisions disrupt the spiral arms in spiral galaxies, which are believed to be the chief cause of star formation, thereby bringing star formation to a premature end despite the existence of fuel[ix]). This leads to a scenario where the universe has become a collection of meta-galaxies, each containing smaller stars slowly burning through their life cycles[x], as well as the remnants of expired stars. Collisions of brown dwarfs (which have been hoarding solar fuel) will trigger bursts of star formation, but these too will burn out, leaving the galaxies much darker places. As the galaxies combine, there is a tendency to think that all the matter within them will also combine, turning galaxies into enormous black holes. However, over the large time periods these processes take, objects within the galaxies will encounter one and other, imparting momentum, and sending many objects in different trajectories, at increased speed (larger objects in the interactions will tend to loose momentum, whilst smaller objects will gain it[xi]). This energy transfer will allow many objects to achieve escape velocity, and leave the meta-galaxy, to await their fate alone in inter-galactic space. It is estimated that over 99% of matter within a galaxy will be lost this way[xii] (planets and other objects orbiting stars and stellar remnants will be similarly ejected from their systems). White dwarfs that escape will slowly radiate heat into the surrounding universe, eventually cooling to thermal equilibrium (becoming black dwarfs). Those objects that do not escape the galaxy, however, will indeed be captured by ever-growing black holes at the centre of meta-galaxies, which will arise from the large black holes believed to exist at the core of most galaxies. At this point the universe consists of large black holes and inter-galactic objects, neither of which are undergoing nuclear processes, and may be considered fairly stable objects. It is at this point that we may feel that the universe has come to an end. However, ‘forever’ is a very long time, and the unlimited supply of time that the future provides allows processes that previously not had sufficient time (or the right conditions) to operate, to start influencing the universe. Also, under the assumption of a open universe, the universe will be much largely they its present state, and significantly, much colder than the 3° K of the present universe. Such large time scales cause unlikely events to become more probable, and one such event is the interaction of exotic particles predicted to contribute to the unseen mass of the universe. These particles are referred to as WIMPS (Weakly Interacting Massive ParticleS). Because the particles have not been directly detected, they remain theoretical constructs[xiii], and theories of their future are obviously speculative. It is believed that over time, these particles will be captured in the centre of stellar remnants, and annihilate with each other, causing their mass to be released as energy[xiv]. In the generous time allowances of an open future, particle reactions whose time scales exceed the current age of the universe start to impact the universe. Chief among these is proton decay. Whilst controversial[xv], proton decay - whereby a proton decays into a positron (positively charged anti-particle to the electron) and a pi meson (an unstable elementary particle that rapidly decays into a pair of photons[xvi]) - is predicted by some particle theories[xvii] ( a minimum half-life[xviii] for this process of 1032 years has been established by earth-based experiments[xix]). This process would impact all baryonic matter in the universe (protons and neutrons), nearly all matter outside black holes[xx]. In matter-rich environments (such as black dwarfs and neutron stars), the positrons will rapidly encounter an electron[xxi], annihilating into two photons. Outside such environments, positrons may exist for extended periods (the extremely low density of the expanding universe reducing the likelihood of an electron encounter). Over time, proton decay would see all baryonic matter converted to energy and non-baryonic particles (primarily electrons and neutrinos). Neutrons are only stable in the nucleus of an atom (and would be released from an atomic nucleus where a proton has undergone decay), where they would decay[xxii] into a proton, an electron and a neutrino – with the proton undergoing subsequent decay. So eventually (estimates put this era at 1060 years[xxiii]) we are left with a universe containing a large number of black holes as well as electrons and positrons, neutrinos and background radiation from various sources. And yet, further capacity for activity within the universe remains. Although black holes appear a one way street for matter (and indeed energy, as black holes will feed off background radiation, and grow in mass – mass and energy being related by Einstein’s general theory of relativity – E=MC2), a quirk of quantum physics allows (indeed mandates) black holes to lose mass. The requirement for this mass loss is that the surrounding universe be colder than the black hole. These conditions will take some time to occur, as black holes are very cold objects (6 x 10-8 K for a one solar mass black hole[xxiv]), and paradoxically, decrease in temperature as they get larger, meaning such mass loss will not occur till after the disappearance of the baryonic universe. Heisenberg’s uncertainty principle (‘the more precisely the position is determined, the less precisely the momentum is known’[xxv], meaning that our understanding of the state of matter relies on averages, rather than true knowledge) allows virtual particles to be created, although they must then annihilate to repay the energy debt used to create them. If such particle pairs are created near the event horizon of a black hole, they may utilise energy from the black hole to repay the energy debt, and become actual particles. This energy extraction is equivalent to mass loss (once again via E=MC2), and over time the black hole ‘evaporates’ via this mechanism (named Hawking radiation, after Stephen Hawking who first proposed it). Because such virtual particle creation at the correct point is so unlikely, this process takes a very long time[xxvi], but it has been shown eventually all black holes will evaporate via Hawking radiation[xxvii]. After 10100 years, all black holes will have evaporated, leaving electrons and positrons and a background of very cold thermal radiation (the expansion of the universe stretches the wavelengths of the photons, reducing their energy levels), as well as exotic particles such as neutrinos. Those electrons and positrons that do not annihilate merge to form exotic atoms called positronium. Unlike regular atoms, these particles are incredibly large (with radii of up to 109 light years[xxviii]) , and circle each other very slowly. However, their rotation orbits gradually decrease, the particle pairs will eventually join and annihilate, leaving a universe on uniform temperature, with no active processes talking place, and no capacity to initiate them[xxix]. We have now reached a state where the universe consists of a uniform sea of photons and neutrinos, a fraction above absolute zero in temperature. More importantly, the universe has reached its most simple and disordered state, and has maximised it entropy. In the battle between entropy and gravity, entropy has won the day, even if that day lasted the entire length of the universe.
Bibliography
Adams, F., Laughlin, G. “The Five Ages of the Universe”, Simon & Schuster, 1999 Kaufmann W, Freedman R, “Universe”, W H Freeman & Company, 1999
[i] see http://www.brookscole.com/physics_d/special_features/in_the_news.html [ii] In a low-metal environment, objects with a mass of less than 0.08 solar masses will remain a brown dwarf, whilst higher masses will achieve high enough core temperature to start nuclear fusion in their cores. Objects with higher metal (non-H or He) contents will be able to achieve higher densities with lower masses, and can achieve nuclear burning with masses as low as 0.04 solar masses – Adams et al, p 42 [iii] 8 solar masses represents the lower limit of a star that has sufficient mass to trigger super novae – Kaufmann et al, p.550 [iv] Kaufmann et al, p.544 [v] Exotic matter, such as WIMPS (see below) also exist, but at this stage, do not interact noticeably with the galaxy, apart from gravitational influences on a galaxy’s rotation [vi] Adams et al, p 60 [vii] Particularly small stars (0.08-0.25 solar masses) do not become white dwarfs, but rather simply burn their fuel and become planet like objects – Adams et al, p60. [viii] Adams et al, p.85 [ix] Baez J, “The End of the Universe”, July 26, 2000, http://math.ucr.edu/home/baez/end.html [x] M31 and the Milky Way are predicted to collide in 6 x 109 years (Adams et al, p84), well within the lifespan low mass stars, however complete aggregation of a cluster is likely to take 1019 years (Adams et al, p87) [xi] Adams et al, p85 [xii] Adams et al, p87 [xiii] …although highly important to explaining many astronomical phenomenon such as galactic rotation [xiv] Adams et al., p91-92 [xv] Proton decay suffers from two main objections – firstly, it has never been observed, and secondly, it violates the law of baryonic conversion. The latter objection (derived from the law that the universe should contain the same number of baryons – a proton disappearing violates this law) has been countered by the theory that the imbalance of matter over anti-matter during the big-bang means that eventually allow protons must decay to redress this balance. [xvi] …or occasionally a positron-electron pair, which are likely to annihilate each other, giving off two photons anyway - http://www.factmonster.com/ce6/sci/A0839136.html [xvii] see http://hep.bu.edu/~superk/pdk.html [xviii] the period of time that half a quantity of matter would undergo this process [xix] see http://www.hep.umn.edu/soudan/brochure.html [xx] within their event horizons – the radius at which the curvature of space-time due to their gravity energy exceeds the speed of light – processes that may occur will not impact the outside universe, and can be ignored [xxi] neutron stars are composed of predominantly of degenerate neutrons, but away from the core, as pressure reduces, non-degenerate matter exists - http://www.astro.umd.edu/~miller/nstar.html [xxii] Unless absorbed into another particle [xxiii] Adams et al, p.238 [xxiv] Baez et al [xxv] http://www.aip.org/history/heisenberg [xxvi] Between 1066 and 1099 year – Baez et al [xxvii] A sufficiently large black hole may never be warmer than the surrounding universe, but evidence exists against such objects, see Dalal N, Griest K, “Black Holes Must Die”, http://xxx.lanl.gov/abs/astro-ph/0008260 [xxviii] Adams et al, p166 [xxix] It is proposed however, that the positronium annihilation process never ceases, and that any uniformity in the universe is localised. The rate of annihilation decreases, but never reaches zero – Adams et al, p.168
